There is concern about the abuse and long term use of benzodiazepine anxiolytic/hypnotic drugs by polydrug abusers and patients. One of the most insidious adverse effects of benzodiazepines is a profound impairment in the ability to form memories of personally experienced events (episodic memory encoding). This project will characterize the changes in brain activity reliably associated with benzodiazepine-induced impairment of episodic memory encoding by parametrically manipulating the level of impairment, and will examine the pharmacological and neurochemical mechanisms underlying these changes. Three double blind, placebo-controlled, within-subject outpatient studies in healthy volunteers are proposed, employing well-established methods in cognitive neuroscience. Following acute drug administration, brain activity associated with performance of a verbal episodic memory encoding task will be measured by regional cerebral blood flow (rCBF) using positron emission tomography (PET) with 15O-H20. Experiment 1 will manipulate the level of encoding impairment via administration of three dose levels of the benzodiazepine hypnotic triazolam. Experiment 2 will manipulate the level of encoding impairment via conjoint administration of triazolam and the benzodiazepine receptor specific antagonist flumazenil, which has been shown to reverse benzodiazepine-induced amnesia; this experiment will also provide information about the pharmacological mechanisms underlying benzodiazepine-induced changes in rCBF during encoding. Experiment 3 will provide information about the neurochemical specificity of benzodiazepine-induced changes in rCBF during encoding by comparing the pattern of rCBF changes produced by triazolam to that produced by scopolamine, a compound which induces comparable decrements in episodic memory encoding but which acts via different receptor site/neurochemical mechanisms. Results of this project will enhance the understanding of the brain mechanisms underlying a serious adverse effect of a widely prescribed and abused class of drugs. Ultimately, data from this research may contribute to the development of new classes of anxiolytic/hypnotic compounds with reduced memory impairing potential, as well as compounds which act to reverse the memory-impairing effects of benzodiazepines. Results of the proposed research also will enhance the understanding of the functional neuroanatomy of basic human memory processes.